Electrical apparatus



Feb. 24, 1953 R. E. PABsT ELECTRICAL 'APPARATUS Filed Dec. 22, 1949INVENTOR Richard EPobs.

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' ATTORNEY Patented Feb. 24, 1953 UNITED srarss PATENT OFFICE ELECTRICALAPPARATUS Richard E. labst, San Antonio, Tex., assigner to WestinghouseElectric Corporation, East Pittsburgh, Pa., a corporation ofPennsylvania Application December 22, 1949, Serial No. 134,460

l Claims. 1

This invention relates generally to electric drives, and more inparticular to an electric drive for controlling the operation of aWindup reel used for winding up the paper coming off a paper mill.

In paper mill drives it is frequently necessary especially in theunreeling and reeling operations required, to provide wide speed rangesof operation for the motors controlling the Winding and unwinding reels.In the usual case a speed range of about 5:1 may be obtained from aconventional direct-current motor by means of ield control. When. theeld is at its full strength the motor is operating at its minimum speed,and when the eld is at minimum strength the inotor is operating atmaximum speed. Variations in speed are also obtainable by armaturecurrent variations, and frequently the motor speed is controlled byutilizing both field control and armature current, or voltage control,to obtain the speed characteristics and speed range which are required.In many cases it is desired on a paper mill drive, in both the unwindingand windup reels, to provide a reel core of some i inches in diameter onwhich the paper is to be wound, the finished diameter being; of theorder of 48 inches, representing a diameter ratio between the full reeland empty reel of about i2zl, indicating a motor speed range of 12:1.

In addition to the above-ncted problems concerning the desirability ofWide speed range control for the motor, it is additionally necessarythat the paper tension be maintained substantially constant in order toprevent overstressing of the paper due to too much tension and possiblebreakage thereof, and undertensioning of the paper which results in thepaper being loosely Wound on the reel. in practice, it has been foundconvenient to utilize the variation in armature current of the motor as'the cue icl' controlling the motor. While armature current variationsare not a precise indication of the strip or paper tension, it has beenfound suliiciently close for practical purposes.

Accordingly, it is a principal object of this invention to provide anelectric drive of the character referred to which aords a Wide speedrange ci Control roi' the motor.

More specically, it is an object of this invention to provide a controlfor a motor to obtain a wide range of motor speeds in which both motorfield excitation control and armature voltage control are utilized tocontrol the motor speed.

It is also an object of this invention to provide a drive for a Windupreel on a paper mill which maintains substantially constant papertension over a wide range of speeds.

More specifically, it is an object of this invention. to provide anelectrical regulating system embodying a control unit capable ofmaintaining an electrical output independently of external control.

The foregoing statements are merely illustrative of the various aims andobjects of this invention. Other objects and advantages Will besoinsapparent upon a study of the following descriptive disclosure whenconsidered in conjunction with the accompanying drawing, in which thesingle figure diagrammatically illustrates an elecn trical controlsystem for a motor utilized to drive the windup reel of a paper mill.

The paper mill in the drawing is only fragmentarily illustrated, thefinal set of calender rolls C being shown and the paper stripterminating in a Windup reel designated WR. The means for driving thecalender roll C is represented in a direct-current motor Ml, thearmature of which is connected in series with the armature of agenerator Gl to be energized thereby. Each of the motor and generator isprovided with eld windings which are energized from a source of directcurrent designated DC. The particular manner in which the excitation iscontrolled in order to obtain constant operating speed is notillustrated in the interest oi' plicity, since per se such equipmentforms no part o this invention. The generator G! is driven by asuitabley induction motor designated l.

The drive for the Wind-up reel WR includes a direct-current motor M, therotor of which is mechanically coupled through suitable gearing (notshown) to the shaft of the windup rcel. Motor M is provided with. aseries-connected interpole iield winding designated MCF and is connectedin series with a :nain generator Gy to energized thereby. The generatoris driven through a suitable mechanical connection with the motor Mlwhich drives the calender rolls. Thus this generator is driven at aspeed proportional to the speed of the calender rolls and with constantexcitation of a predetermined degree determined by the setting oi therheostat Rl in the circuit of the field GF which is energized across the13C supply, the output of the generator G will be a functiszm.v of itsspeed of operation. Accordingly, assuming constant excitation of themotor field MF which is also energized across the Source DC. the speedof the motor M will be controlled by armature voltage variations, andwill accordingly tend to be proportional to the speed of operation ofthe calender rolls C. .As a rule the calender C, once the machine is inoperation, will be operated at substantially constant speed. However,for the purpose of threading the paper through the roll stands, it isnecessary that these rolls be operated at a fairly low speed, and such`speeds are obtained as a rule by properly controlling the excitation ofthe eenerator GI, and ordinarily are such as to afford a paper velocitythrough the stands o about 50 feet per minute. During such operation ofthe mill., the motor M must be operated at a correspondingly low speed,and once the threadingoperation is completed and the paper properlysecured on the windup reel to 'ce wound thereon, the speed of the millis stepped up, and speeds as high as 2,000 feet per minute arecontemplated requiring extremely accurate speed control ci the windupreel to maintain the degree of tension required in the paper.

The control according t0 this invention which accomplishes thisobjective embodies a regulating generator generally designated R. Thisregulating generator when used in regulating systems ador-ds cerainadvantages which are not realizable with conventional exciters. Thisgenerator is of the self-energizing type and its load circuit is soadjusted by means, for example, of an adjustable resistor R2, which inthe instant application is connected in series with the armaturecircuit, that the line of the load circuit is tangent to the initialstraight line portion of the no-load saturation curve. energizing isprovided with a series eld winding designated RSF and is capable ofproducing an electrical output upon rotation thereof due to the residualux independent of external excitation. The output may therefore beanywhere along the tangent curves of one polarity or the reverse,depending on the direction of rotation with respect to the residualflux. While not shown in the drawing in the interest of simplicity, thisgenerator forms part of a small motorgenerator set, and in the instantapplication is intended to be driven at a constant speed. Thus in orderto control the output of this generator, certain intelligences areapplied thereto in the form of excitation provided by a plurality ofcontrol iield windings which are selectively excited frcm differentpoints in the system in order to obtain the type of output which isdesired to regulate the motor M through the main generatei' G so thatits horsepower output will be l substantially constant.

Accordingly, the field system of this generator includes a main fieldwinding RPF which may be identified as the pattern eld winding. This`field winding is excited from the directurrent source DC through aseries-connected rheostat R3 which is adjusted to provide the requiredmagnitude of current for the main excitation of the regulatinggenerator. A current field winding RCF .responsive to the motor armaturecurrent is diierentially` related to the pattern held winding RPF asshown by the arrows adjacent the respective windings. This current fieldis excited by means of a generator IR which in turn is excited by a neldIRF connected across the interpole winding MCF of the motor to respondto the voltage thereacross. The voltage of this interpole field winding,it will be appreciated, isproportional to the motor armaturecurrentysince This generator to be self- I the armature current flowstherethrough. The final eld winding for the regulating generatordesignated RDF and is also diierentially related to the pattern fieldwinding RPF as shown by the arrows adjacent the respective windings.This leld winding is energized in dependence of the voltage drop acrossthe generator field winding GFZ which in turn is excited by theregulating generator. In accomplishing this, the terminals of the eldwinding GFZ are connected across the armature circuit of the regulatinggenerator in a circuit including a rectier RCI. The differential eldwinding RDF is connected across the generator field winding GF2 in acircuit which includes a rheostat R4 which ls mechanically ganged withthe rheostat R3 so that adjustments of the pattern field windingexcitation by means of the rheostat R3 will simultaneously vary theexcitation of the differential eld winding RDF1 to maintain the desiredproportionality of the differential excitation of this differentialfield with respect to the pattern neld RPF.

The rnotor M is provided with an additional neld winding designated MFZwhich is cumulatively related to the main iield winding for the motorMFI. This eld winding is connected across the armature circuit of theregulating generator in parallel with the generator eld GF2 in a circuitwhich includes a rectifier RC2.

The generator eld winding GFZ for the polarity of excitation ailorded bythe rectifier RCI is cumulatively related to the main generator fieldwinding GFI. Thus the eld GFZ when excited adds its excitation to thatof the generator field GFI to increase the output of the generator, andwhen the polarity of the output voltage of the regulating generatorreverses, this field is no longer excited, because the excitationvoltage is applied in opposition to the rectifier RCI which thereforeblocks a current ilow through the eld GP2.

The motor iield MFZ which is cumulative with respect to the motor fieldMFI is prevented from being excited by that polarity of voltage whichexcites the generator eld GFZ because the rectier RC2 in series in thismotor field circuit is in cppositon to the current flow, but upon areversal in the polarity of output of the regulating generator currentflow through the rectifier RC2. and the Iield MFZ is excited adding itsampere turns to those of the main field and further reducing the speedof the motor below that afforded by armature voltage control alone.

The regulating generator is controlled by its nelds as follows: Theoutput of the regulating generator upon rotation thereof and upon theapplication of the pattern field winding excitation tends to increase inthe direction urged by the pattern eld excitation. The series neldexcitation for this condition is indicated by the solid arrow adjacentthereto. The ampere turns of the two differential field windings RCE1and RDF subtracts from those of the pattern iield winding, and theseiields are so designed that for a predetermined degree of excitingcurrent their ampere turns will be equal and opposite to those of thepattern field windng, at which time the total external excitation of themachine is zero. If the machine tends to increase its output beyond thecondition at which the pattern and differential nelds are balanced, thedifferential excitation will be increased due to changes in theelectrical conditions in the system to which these elds are responsive.As a result of the increase in the differential excitation, the machine.is excited in an opposite sense tending to reduce its output to thatwhich is required for proper control of the system. On the other hand,if the output for any reason is insufficient, or for any reason tends tofall below that required for an instant condition in the system, theelectrical conditions of the system reflected in a dropping differentialexcitation of the machine results in a net excitation in favor of thepattern field tending to urge the output of the generator upwardly, atwhich time the increasing differential excitation due to theprogressively changing electrical conditions in the system again reducesthe net external excitation to zero. It will therefore be appreciatedthat the function of the pattern and differential fields of thegenerator is that of urging the output of the generator in the properdirection and to the proper magnitude, at which time the effect of thesefields is balanced or reduced to zero and the generator, due to itsself-energizing properties, maintains its electrical output until achange in the conditions in the system requires a different output.

This invention will be better understood from a consideration of theoperation of this system. Inasmuch as the function of the inventionduring threading operation is similar in some respects to that for acontinuous windup operation, insofar as the indication for correction ofsystem errors is concerned, the following description will be confinedto a constant paper speed reeling operation. Assuming that the papermill is operating at normal speed, the main generator G will produce anelectrical output Which is proportional to the speed at which the paperis passing through the calendar rolls. This voltage, which is applied tothe armature of the reel motor M which is constantly excited by itsfield MFI, results in a motor speed which is proportional to the paperspeed. However, with each revolution of the wind-up reel, the reeldiameter is increased by the thickness of the material being woundthereon. Therefore, the reel radius is increasing andthe revolutions perminute must be reduced in order t0 maintain a constant peripheralvelocity corresponding to the linear speed of the paper. As the reeldiameter increases the motor tends to slow down due to the increasingtorque loading. As a consequence for a given degree of excitation of themain motor the armature current will tend to increase due to the factthat the back EMF of the motor is decreasing. As the armature currentincreases the excitation of the regulating generator control field RCFalso tends to increase.

Assuming further that the windup operation has just begun, the polarityof output of the regulating generator is now such that current flowsthrough the field GF2 since the polarity of this voltage is such thatthe rectifier RCI will pass current. The polarity of the series fieldRSF for this condition is indicated by the solid arrow. Thus the fieldRDF of the regulating generator is energized by the voltage across thegenerator field GFZ, and atV this instant its .ampere turns added tothose of the field RCF are momentarily balanced against those of thepattern field RPF, but the armature current is increasing in thisassumption,` and therefore the differential excitation is increasing andpredominates that due to the pattern field RPF. As a consequence, theoutput voltage of the regulating. generatorY is. decreased- -reducingtherv excitation of the field GFZ, and correspondingly reducing theoutput voltage of the main generator to tend to reduce the motor speed.This results in a drop in the excitation of the differential field RDFand the differential and pattern field excitation again tends toapproach equilibrium when the regulating generator output corresponds tothat which is required by the system. Thus it will be appreciated thatthe output of the regulating generator from that value which 'affordsexcitation of the eld GF2 to give maximum speed is progressively reducedand the system is arranged so that for a given diameter of the reel theregulating generators output Will be reduced to zero, and consequentlythe field GFZ will no longer contribute excitation to the generator G.The speed control range is noW further varied in response to furtherchanges in reel diameter and motor armature 'current by increasing thetotal excitation of the motor M. Further increases in armature currentagain increase the differential excitation which now drives theregulating generator through Zero output and causes it to build up itsoutput inthe reverse direction. The polarity of the series field RSF isnow reversed as indicated by the dotted arrow. This reverse outputvoltageA is now blocked by the rectifier RCi and the generator field GFZis therefore not excited and likewise field RDF of the regulatinggenerator is not excited. But the polarity of this reversed voltage issuch that the current thereof is passed by the rectier RC2 energizingthe motor field MF2 which adds its excitation to that of the field MFIstrengthening the motor field and further reducing the speed thereof.This operation continues as described until the maximum reel diameterhas been reached.

In a practical example upon completion of the threading operation thesupercalender roll system is brought up to operating speed and the reelmotor operates at maximum speed, as determined by minimum reel diameter.At this point in the windup operation the main generator voltage isapproximately twice in value that which would be vgiven by the constantvalue main generator field GFL The additional excitation to obtain thisdouble voltage is supplied byv the generator auxiliary field GF2through' its blocking rectifier RCi.` The valueof regulating 'generatorvoltage needed to provide `the required lexcitation of eld GFZ isobtained by the resultant `interaction of the regulating generatorfields RPF, RCF and RDF. Forthiscondition of maximum speed, fieldA RDFis so proportioned that when added to RCF the sum of the differentialampere turns equals those of RPF. The regulating generator yvoltagetherefore builds up in the direction urged by pattern field RPF untilthe control excitation reaches zero withrectiiier RC2 preventing motorfield MFE being differential with respect to MFI. The value of motorcurrent at maximumspeed which corresponds to vminimum reel diameter willbe approximately one-half the constant value over the slow speed range.As the reel builds up in diameter; the motor speed decreasesashereinabove noted with an increase in motor current resulting in--anincrease in 'the excitation of the differential current field of theregulating generatonfreducing this generators output `which in turnldecreases `the main voltage. vAs the speed decreases it will beappreciated that the'motor current continues to increase until theregulating generatorvoltage is zero and 'the-main generator Vvoltage isroughly one-half the initial value. The speed range of adjustmentafforded by armature voltage control is approximately 2: 1, and duringthis control the motor current will increase linearly from 50% to 100%,While the main generator voltage decreases from 200% to 100% giving aresulting horsepower over the speed range which will vary from thedesired value at maximum speed to approximately U21/2% of that value at'75% speed and back to 100% at about one-half speed. From one-half speeddown to minimum speed the control of this invention will function withthe regulating generator exciting the ield MF2 cumulatively with respectto MFI. while the blocking rectifier RCI prevents any further effectfrom the differential eld RDF of the regulating generator and the fieldGF2 of the main generator.

The adjustment ailorded by the rheostat R3 in series with the regulatinggenerator pattern eld RPF provides for the recalibration of papertension. When tension is recalibrated by adjustment of the patternfield, the mechanical ccnnection between rheostats R3 and RIIautomatically recalibrates the field RDF' to maintain a xed ratio ofexcitation between these fields.

When the speed of the paper is changed by changing the speed of themotor MI, the main generator voltage which is linear with the generatorspeed will correspondingly change and result in the reouired motorspeed. The paper tension under this condition will remain, the samesince it is determined by the motor current and the regulating generatorcontrol which is unchanged in this change of paper speed, The maingenerator eld GFI remains unchanged. Thus the relation between theilelds GP2 and GFI will remain unchanged by the various speed ranges ofthe calender rolls afforded by adjustment of the speed of motor MI.

While but one embodiment oi this invention has been herein illustratedand described, it will be appreciated by those skilled in the art thatnumerous variations in control details may be obtained without departingfrom the spirit and scope of the teachings of this invention.Accordingly, it is intended that the foregoing disclosure shall beconsidered only as illustrative and not interpreted in a limiting sense.

I claim as my invention:

1. In an electric drive for controlling the speed of a motor over a widespeed range, the combination of, a motor having a pair ci cumulativelyrelated feld windings, a main generator having a pair of cumulativeileld windings, circuit means for applying a predetermined degree ofexcitation to one motor field winding, circuit means for applying apredetermined degree of excitation to one generator iield winding,circuit means connecting said main generator to said motor to energizesaid motor, a regula-ting generator, excitation circuit means forapplying a predetermined degree of excitation to said regulatinggenerator, circuit means including a series-connected rectier connectingsaid regulating generator to the other generator field winding to excitesaid other generator neld winding cumulatively with respect to said onegenerator field winding, circuit means including a series-connectedrectifier for connecting said regulating generator to the other motoreld winding to excite said other motor eld winding cumulatively withrespect to said one motor field winding and to prevent excitation ofsaid other motor ileld winding for the polarity of voltage output ofsaid regulating generator used to excite said other generator fieldwinding, and circuit means responsive to the current supplied to saidmotor for exciting said regulating generator differentially with respectto said excitation circuit means.

2. In an electric drive for controlling the speed of a motor over a widespeed range, the combination of, a motor having a pair of cumulativelyrelated eld windings, a main generator having a pair of cumulative fieldwindings, circuit means for applying a predetermined degree ofexcitation to one motor field winding, circuit means for applying apredetermined degree of excitation to one generator eld winding, circuitmeans connecting said main generator to said motor to energize saidmotor, a regulating generator, excitation circuit means for applying apredetermined degree of excitation to said regulating generator, circuitmeans including a series-connected rectiiier connecting said regulatinggenerator to the other generator eld winding to excite said othergenerator iield winding cumulatively with respect to said one generatorfield winding, circuit means including a series-connected rectier forconnecting said regulating generator to the other motor field winding toexcite said other motor field winding cumulatively with respect to saidone motor field winding and to prevent excitation of said other motoriield Winding for the polarity of voltage output of said regulatinggenerator used to excite said other generator eld Winding, circuit meansresponsive to the current supplied to said motor for exciting saidregulating generator differentially with respect to said excitationcircuit means, and circuit means responsive to the voltage of said othergenerator eld winding for further differentially exciting saidregulating generator.

3. In an electric drive for controlling the speed of a motor over a widespeed range, the combination of, a main generator for energizing themotor, circuit means for applying a predetermined degree of excitationto said main generator, circuit means ior applying a predetermineddegree of excitation to said motor, a regulating generator, circuitmeans responsive to the output of said regulating generator' forcumulatively exciting said main generator for one polarity of saidoutput and for preventing said last-named excitation of said maingenerator for an opposite polarity output of said regulating generator,circuit means responsive to the output of said regulating generator forcumulatively exciting said motoi` in dependence of said oppositepolarity output and for preventing said cumulative excitation of saidmotor during said one polarity of output of said regulating generator,and circuit means responsive to the current of said motor forcontrolling said regulating generator.

e. Apparatus as set forth in claim 3 in which said regulating generatoris a self-energizing generator adjusted to operate on its air-gap line.

5. In an electric drive for controlling the speed of a motor having apair of field windings, the combination of, a main generator having apair of eld windings, circuit means for connecting said main generatorto said motor to energize said motor, circuit means for applying apredetermined degree of excitation to one field Winding of saidgenerator and one field Winding of said motor, a regulating generator,circuit means responsive to motor current for controlling saidregulating generator, circuit means responsive to one polarity of outputof said regulating generator vfor exciting'the other eld windingof-saidlmain generator, and circuit lmeans responsive to a ree versal in outputof said regulating generator for exciting the other -neld windingof'said motor.

6. In an electric drive for controlling the speed of amotorhaving a pairof iieldwindings, the combination of, a main generator having a pair cield windings, circuit means for connecting said main generator to saidmotor'to energize said motorfcircuit means for applying a predetermineddegree of excitation to one riield winding of'saidY generator and onefield winding of said motor, a regulating generator, said regulatingAgenerator being a self-energizing generator adjusted to operate on itsair-gap line, circuit means responsive to motor current for controllingsaid lregulating generator, circuit `rneans responsive to `onetpola'rityof .output of Vsaid regulating generator for exciting the other fieldwinding oi said mainlgenerator, and circuit means responsive to areversal in output of said regulating generator for exciting the otherneld winding of said motor. 7. In an electric drive for controlling thespeed fof aimotor'having apair of-field windings, the combination of, amain generator having a pair of field windings, circuit means for`connecting said main generator to said motor to energize said motor,circuit means for applying a prede terminededegree ofexcitation'to onefield Wind ing of said generator and one neld winding oi said motor, aregulating generator, a main field winding and a diierential fieldWinding for the regulating generator, circuit means for applyingpredetermined excitation to said main eld winding, circuit meansresponsive to motor current for exciting the differential field windingoi the regulating generator, circuit means responsive to one polarity ofoutput of said regulating gen erator for exciting the other eld windingof said main generator, and circuit means responsive to a reversal inoutput of said regulating generator for exciting the other field windingof said motor. S. In an electric drive for controlling the speed of amotor having a pair of cumulatively related eld windings, thecombination of, a generator having a pair of cumulatively related iieldwindings and having circuit connections for connection to said motor toenergize said motor, circuit connections for applying a predetermineddegree of excitation to one iield winding of said motor, circuitconnections for applying a prede termined degree of excitation to onenield winding of said generator, amplifier means having an outputcircuit, control means for said ampli-ner means for aording one polarityof output of said amplifier means, control means ior said ampliner meansresponsive to an operating quantity ci said motor for controlling saidampliiier means in reverse the polarity of output thereof, circuit meansconnecting said output circuit with the other iield winding oi saidgenerator for energizing said other field winding of said generator forone polarity of output of said amplier means and preventing excitationof said other field winding of said generator upon a reversal inpolarity of output of said ampliiier means, and circuit means connectingsaid output circuit with the other iield winding of said motor forprevent ing energization of said other field winding of said motor forsaid one polarity of output of said amplifier and for energizing' saidother eld winding of said motor upon a reversal in polarity of output oisaid amplifier means.

9. In an electric drive for controlling the speed of a motor having apair of cumulatively re- "lated iield windings, the combination of,4 agenerator having a pair of cumulatively related iield windings andhaving circuit connections for connection to said motor to energize saidmotor, circuit connections for applying a predetermined degree ofexcitation to one eld winding of said motor, circuit connections forapplying a predetermined degree of excitation to one ield winding ofsaid generator, ampliiier means having an output circuit, control meansfor said amplier means for aording one polarity of output of saidamplifier means, control means for said amplifier means responsive to anoperating quantity of said motor for controlling said aniplier means toreverse the polarity ci output thereof, circuit means connecting saidoutput circuit with the other eld winding of said generator forenergizing' said other field winding of said generator for one polarityof output of said amplifiermeans and preventing excitation of said othereld winding of said generator upon a reversal in polarity of output ofsaid amplifier means, circuit means connecting said output circuit withthe other field winding of said motor for preventing energization ofsaid otheriield winding of said motor '.ior saidA one polarity o outputor said ampliand for other Yeldwinding S ofsaid motor'upon a reversalyin polarity of output of said amplifier means, and control meansresponsive to the voltage of sai-d other Iield winding of said generatorfor additionally controlling said ainpliier means to reverse thepolarity of output thereof.

10. In a control for a motor-generator drive wherein the armature of thegenerator is electrically connected to the armature of the motor, themotor and the generator being separately excited and each having acontrol field winding, the combination of, amplier means having anoutput circuit, control circuit means for controlling said arnplinermeans to produce one polarity of electrical output, control circuitmeans for connection with said motor to respond to an operating quantitythereof for controlling said amplifier means to reverse the polarity ofoutput thereof, circuit connections including rectifier means forconnecting the control eld winding of the generator to said outputcircuit to energize said control field winding of said generator forsaid one polarity of output of said amplier means, and circuitconnections including rectiier means for connection to the control neldwinding of the motor for energizing the control field winding of themotor upon reversal of the polarity of output or" said amplifier means.

1l. Apparatus as recited in claim 10, wherein said amplifier means is aself-energizing generator having a load circuit resistance causingoperation of the self-energizing generator on its air-gap line.

l2. In a control for a motor-generator drive wherein the armature of thegenerator is electrically connected to the armature of the motor, themotor and the generator seing separately excited and each having acontrol neld winding, the combination of, ampliiier means having anoutput circuit, control circuit means for controlling said ampliiermeans to produce one polarity of electrical output, control circuitmeans for connection with said motor to respond to an operating quantitythereof for controlling said amplifier means to reverse the po larity ofoutput thereof, circuit connections including rectier means forconnecting the control eld winding of the generator to said outputcircuit to energize said control field winding of said generator forsaid one polarity of output of said amplier means, circuit connectionsincluding rectifier means for connection to the control field winding ofthe motor for energizing the control field Winding of the motor uponreversal of the polarity of output of said ampliiier means, and controlcircuit means for connection to the control iield winding of thegenerator to respond to the voltage thereof for additionally controllingsaid amplifier means to reverse the polarity of output thereof.

13. Apparatus as recited in claim 12 in which said amplifier means is aself-energizing generator having a load circuit resistance causingoperation of the self-energizing generator on its air-gap line.

14. In an electric drive for controlling the speed of a motor having apair of field windings, the combination of, a main generator having apair of field windings, circuit means for connecting said main generatorto said motor to energize said motor, circuit means for applying apredetermined degree of excitation to one field winding of saidgenerator and one eld winding of said motor, a regulating generator, amain eld winding and a pair of differential eld windings each of whichis differentially related to said main field winding of said regulatinggenerator, circuit means for applying predetermined excitation to saidmain eld Winding, circuit means responsive to motor current for excitingone differential field Winding, circuit means responsive to one polarityof output of the regulating generator for exciting the other eld Windingof said main generator, circuit means responsive to a reversal in outputof the regulating generator for exciting the other field winding of themotor, and circuit means connecting the other differential field Windingof the regulating generator to said other field winding of said maingenerator to be energized in dependence of the voltage thereof.

15. Apparatus as recited in claim 14 and in addition, means forsimultaneously varying the excitation of said main field winding andsaid other differential field winding of said regulating generator inthe same sense.

RICHARD E. PABST. l'

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,406,426 King Aug. 27, 1946

